Investigators are no closer to determining the source of an outbreak of E. colithan they were when the first of 64 people in the Northeast became ill in early November, federal health officials said Monday.

Tests on green onions, believed to have been a possible cause, were negative, they said.

But the outbreak, linked to Taco Bell restaurants, may be winding down. No new cases have been added to those reported in five states since late last week, the U.S. Centers for Disease Control and Prevention reported.

Dr. Christopher Braden, an epidemiologist with the U.S. Centers for Disease Control and Prevention, told a news teleconference there may yet still be unconfirmed cases of people sickened by E. coli.